Atypical seasonal variability of the Kuroshio Current affected by intraseasonal signals at its origin based on direct mooring observations

The spatial distribution and temporal variability of the Kuroshio Current (KC) was investigated with three moorings deployed at 122.7° E, 123° E, and 123.3° E along 18° N from January 2018 to the spring of 2020. It is shown that the core of the KC is located to the west of 122.7° E along 18° N. With the increase in longitude, the KC extended its vertical scale and attenuated its intensity gradually. The satellite data indicated that the KC was strongest in winter and spring, while it was weakest in autumn along 18° N. However, the seasonal cycle of the KC from mooring observations was atypical compared with that from the satellite data. The seasonal variation of the KC was not obvious in 2018, and a summer peak of KC occurred in 2019. The atypical seasonal variability of the KC was attributed to the strong intraseasonal signals generated by eddy activity. Eddies propagated from east and were enhanced to the west of 140° E, leading to the westward intensified intraseasonal signals. In addition, the intraseasonal signals varied interannually, that is why the variation of the KC in 2018 was quite different with that in 2019.

www.nature.com/scientificreports/ North Pacific Subtropical Countercurrent (STCC) and the NEC system [34][35][36] . It has been shown that anticyclonic (cyclonic) eddies strengthen (weaken) the transport of the KC to the east of Luzon and Taiwan 37 .
Although there are some mooring measurements for the KC to the east of Taiwan (e.g., Refs. [38][39][40][41] ), long-term direct observations from mooring arrays in the source region of the KC are relatively sparse (e.g., Refs. 10,29,42 . Three moorings at 122.7° E, 123° E, and 123.3° E along 18° N are utilized to discuss the mean structure, seasonal variability, intraseasonal variability of the KC, and then to clarify the influence of intraseasonal variability on the seasonal cycle of the KC in the present study.

Data and methods
In January 2018, three moorings were deployed at 122.7° E, 123° E, and 123.3° E along 18° N to the east of the Philippines with the R/V Science (Fig. 1). Each mooring had an upward looking acoustic doppler current profiler (ADCP) and a downward looking ADCP. The moorings were then retrieved successfully in May 2020 with the R/V Science 3 during a research voyage of the Institute of Oceanology, Chinese Academy of Sciences (IOCAS). The three moorings were designed to clarify the spatial distribution and temporal variability of the KC/LUC as well as the role of eddy activity in this region where the water depths are 3124 m, 3383 m, and 3437 m at 122.7° E, 123° E, and 123.3° E along 18° N, respectively. Unfortunately, one of the downward-looking ADCPs at longitude 123° E was broken. Table 1 provides information about the three moorings.
In detail, upward and downward looking 75 kHz ADCPs were fixed on the main float of each mooring. Each ADCP was set as 60 bins with 8 m per bin; thus, its measurement range was 480 m. As the main float was designed at 450 m, the velocities in the upper 900 m could be well monitored by two ADCPs. We note that the strength of the KC led to an incline of the mooring system, suggesting that the real measurement range was < 900 m. The mooring data with 8 m intervals were interpolated into 1 m resolution. For data quality control, the velocity magnitude was limited to 3 m/s, the percent good beam in ADCPs with a quality of > 80 were accepted, and the data with > 18° of pitch and roll were deleted. We note that the velocities in the upper 50 m of the water column were deleted owing to poor quality data that resulted from the strong echo of beam emission. The velocity resolution of ADCP is 1 mm/s, and the velocity accuracy is ± 1% ± 5 mm/s. As the ADCP data were collected on an hourly basis, it was convenient to convert the mooring data to a daily time series to remove tidal signals.

Results
Mean structure of the KC. Around 18° N, the direction of the KC is almost parallel to the coastline of Luzon Island (Fig. 1)    This suggests that the intraseasonal signals varied interannually, which is consistent with the above analysis. We note that the magnitude of the period of 50-60 days attenuated from 122.7° E to 123.3° E, which indicates that the intraseasonal signals are stronger towards the west. It is noted that the inraseasonal signals were significant below 400 m at 123.3° E in 2018. The temporal and spatial resolutions of the mooring data were insufficient and limited to 2 years and three stations. Our calculation shows that the geostrophic velocities were very consistent with the mooring observations averaged over the upper 150 m, both with respect to amplitude and phase (Fig. 5). These findings suggest that satellite altimeter data are convenient for investigating the seasonal and intraseasonal variability of the KC. Moreover, these findings also indicate that the direct observations could be considered as geostrophic currents in the origin region of the KC. This is consistent with the results of Lien et al. 29 , who found that the geostrophic theory could be used to describe the KC (except in the upper 100 m of the KC's western flank).
In Fig. 6a, the mean monthly meridional velocities based on satellite data indicate that the KC was strongest in winter and spring, and weakest in autumn during the period from 1993 to 2019. Notice that the seasonal variation of the KC is different along Luzon Strait. For example, Zhang et al. 27 indicates a July maximum of the KC in the northern flank of Luzon Strait. The mean monthly sea surface height anomaly (SSHa) averaged from 1993 to 2019 is shown in Fig. 6b. Two adjacent zonal points ( x 1 and x 2 ) in the origin region of the KC along 18° N were chosen to discuss the phase difference of the zonal SSHa. The maximum SSHa was in August at 121° E, while it occurred in July at ~ 123.7° E, so we set x 1 = 121° E and x 2 = 123.7° E. As the propagation speed of Rossby waves near the western boundary along 18° N is approximately 0.11 m/s 43 , t = x 2 −x 1 c r ≈ 1 month. This result is consistent with the fact that the maximum SSHa at 123.7° E required nearly 1 month to arrive at 121° E, as shown in Fig. 6b, suggesting that Rossby waves process is one of the possible dynamical processes in the seasonal variability of the KC. At all mooring stations, the zonal gradient of the SSHa was always positive from December to July (Fig. 6b), generating a northward current anomaly that increased the KC. In contrast, the zonal gradient of the SSHa was negative from August to November, corresponding to a southward current anomaly that decreased www.nature.com/scientificreports/ the KC. In addition, the mean monthly SSHa across the Pacific Ocean along 18° N is depicted in Fig. 6c. It is worth emphasizing that the spatial distribution of the SSHa along 18° N differed considerably to the east and west of the dateline. To the east of the dateline (180° E), the positive SSHa occurred during July-December, and the negative SSHa existed during January-June. However, to the west of the dateline, the positive SSHa occurred during May-October, and the negative existed during November-April. This difference may have been strongly influenced by the longitude-dependent spatial distribution of wind forcing in the North Pacific Ocean. Zhang et al. 28 showed that the local wind plays an important role in seasonal upper-layer velocity variations of the KC. Thus, the seasonal KC is the result of the local wind, Rossby waves, surface heat flux 44,45 and so on. However, it is obvious that the seasonal variation derived from satellite data in Fig. 6a is different from mooring observations especially in 2018 in Fig. 3. The meridional velocity anomalies from January 2018 to December 2019 calculated from satellite altimeter data along 18° N are shown in Fig. 6d. It is noted that the magnitude of seasonal variability in Fig. 6a is smaller than the intraseasonal signals in Fig. 6d, suggesting that the intraseasonal variability is more significant than the seasonal variability during mooring observations. In fact, the origin of the KC is a rich eddy region where the intraseasonal variability of the KC is strongly influenced by eddy activity 35,42 . Obviously, that eddies propagated westward and were enhanced to the west of 140° E along 18° N in Fig. 6d, well corresponding to the westward intensified intraseasonal signals. In addition, the behavior of intraseasaonal signals in 2018 were different from that in 2019, indicating that the intraseasonal signals varied interannually. For example, the positive maximum of the SSHa occurred at the mooring stations in September and October 2018, resulting in a strong northward flow anomaly. In 2019, strong anticyclonic eddies from the east arrived at the mooring stations in June and July, thus generating a summer peak of the KC.

Conclusion and discussion
The seasonal and intraseasonal variabilities of the KC in its origin region were investigated using satellite altimeter data and direct mooring observations at 122.7° E, 123° E, and 123.3° E along 18° N. The KC extended its vertical scale and attenuated its intensity gradually with increased longitude. Beneath the permanent northward KC, the  www.nature.com/scientificreports/ intermittent southward LUC led to an increase in the standard deviation of the current velocity below a depth of 400 m. Within the observational time series, the KC often exceeded 1 m/s. The satellite data indicated that the KC was strongest in winter and spring, and weakest in autumn along 18° N. The consistency between the mooring data and the meridional velocities calculated from satellite altimeter data indicates that the KC at the three mooring stations maintained a geostrophic balance during the study period. It is shown that the SSHa gradually propagates toward the west; for example, there is a ~ 1 month interval between 121° E and 123.7° E, thus generating a zonal gradient of the SSHa. This situation leads to a negative zonal gradient from August to November, and a positive gradient from December to July, thereby producing a complete seasonal cycle of the KC.
The mooring observations showed that the seasonal variability of KC was not obvious and the intraseasonal signals played an important role in 2018, which is confirmed from the PSD that the intraseasonal signals were stronger in the upper 350 m in 2018 than in 2019. The origin of the KC is a rich eddy region where the intraseasonal variability of the KC is strongly influenced by eddy activity 35,42 . These eddies propagated from the east and were enhanced to the west of 140° E.
To depict the spatial structure of the SSHa variability more clearly, the SSHa in 2018 in the region of 121°-130° E/15°-20° N is shown in Fig. 7. The arrows represent the corresponding geostrophic current anomalies. It reveals that the cyclonic and anticyclonic eddies simultaneously occupied the region to the east of the mooring  www.nature.com/scientificreports/ stations from March to June, and in December 2018. The mooring stations were located in the western flank of these two eddies. The interaction of these two eddies meant that the northward and southward current anomalies alternately occupied the mooring stations, suggesting a relatively higher frequency compared with that without eddies in the origin region of the KC. Gordon et al. 42 pointed out that there are two energetic dipoles bracketing a northward flowing stream into the KC. The cyclonic dipole occupies the southern tier of Lamon Bay, while the center of the anticyclonic dipole may lie at 17° N/125° E. In September and October in Fig. 7, an anticyclonic circulation occurred in the area to the east of our mooring stations, which almost coincided with the anticyclonic dipole mentioned above, and resulted in a northward current anomaly that increased the KC. Obviously, eddies propagated westward and reduced when they arrived at the western boundary. In this process, intraseasonal variability of the KC is controlled by the propagation and reduction of eddies. Therefore, intraseasonal signals are important and cannot be ignored in the seasonal cycle of eddy-rich years in the origin region of the KC.